The standard chromium plating solution includes chromium trioxide and sulfuric acid dissolved in water. For example, a standard solution might include 33 ounces of chromium trioxide and 0.33 ounces of sulfuric acid per gallon of solution. While this is a common concentration, the chromium trioxide may vary from 20 to 50 ounces per gallon although, in most cases, the 100 to 1 ratio of chromium trioxide to sulfuric acid is maintained. This is called a 1% solution. Whether this or some other percentage is desired for a particular application, the percentage is critical to insure proper and effective plating and must therefore be known. For this reason, certain tests were developed in the prior art to make this measurement.
Perhaps the most common sulfate test, at present, consists of adding barium chloride to a measured amount of plating solution. The sulfate is precipitated out as barium sulfate. The solution containing barium chloride is placed in a test tube and centrifuged to pull the barium sulfate into a small diameter extension in the bottom of the tube. The level of the barium sulfate is read on a scale printed on the tube. Each measurement requires at least two centrifuging operations and several measurements are required to insure accurate results. The test requires about 15 to 20 minutes to perform and its success, of course, depends on the performer's skill.
Another prior art means for making percent sulfate measurements is a device called an LPW sulfometer which is quite old and no longer used, presumably because it was awkward and unreliable. The device includes a couple of electrodes, one of brass and the other of lead, connected through a rheostat to a DC power supply. An ammeter is also connected in the circuit to measure the current through the electrodes. To measure the sulfate content, the brass and lead electrodes are immersed in the solution, and the operator then varies the rheostat gradually at a prescribed rate and as steadily as he can to gradually increase the voltage on the electrodes. As will be further explained later in this application, a point is reached where the current through the electrodes begins to decrease as the voltage increases. This is a peculiar characteristic of the plating solution. The magnitude of this current peak is directly proportional to the sulfate content of the solution. Hence the operator, watching the ammeter mentally notes where the maximum reading occurs and with the meter graduated in percent sulfate, reads the sulfate content of the solution.
There were several problems with the LPW sulfometer device. For one thing, the brass electrode was attacked by the solution each time the test was made and had to be either replaced or repolished after each test. Also the lead electrode had to be cleaned after each test. Another problem was that the electrodes were improperly configured so that the measurements were often inconsistent and unreliable. Additionally, the accuracy of the results depended on the skill of the operator to manually vary a rheosotat at a prescribed rate, which is virtually impossible to do. For this reason, the LPW sulfometer device is generally not used today.
Obviously, neither of these prior art methods or devices offers a completely automatic system for measuring the sulfate content of a chrome plating solution; and, hence, there is no system presently available for automatically controlling the solution. Therefore, it is a primary object of this invention to provide a completely automatic device for making these measurements accurately and in only a few seconds, which device uses components which are not attacked by the solution and therefore permanently re-usable, and which can be used for automatically controlling the solution.